Gas sensor

ABSTRACT

A gas sensor comprises a main pumping cell for pumping-processing oxygen contained in a first chamber, a feedback control system for comparing a partial pressure of oxygen in the first chamber with a first reference value to control the main pumping cell so that the partial pressure of oxygen has a predetermined value at which NO is not decomposable, an auxiliary pumping cell for pumping-processing oxygen in the second chamber, and a measuring pumping cell for pumping-processing oxygen produced by decomposition of NOx. The gas sensor further comprises a correcting control system for correcting and controlling the feedback control system on the basis of a difference between a second reference value and a value of a pumping current flowing through the auxiliary pumping cell to give a constant oxygen concentration in the second chamber, and a self-diagnosis unit for comparing the value of the pumping current with a prescribed range and judging whether or not any trouble occurs, on the basis of an obtained result of comparison. Accordingly, it is possible to provide the gas sensor having a self-diagnosis function capable of quickly and reliably detecting whether or not any trouble occurs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor for measuring oxides suchas NO, N₂, SO₂, CO₂, and H₂ O contained in, for example, atmospheric airand exhaust gas discharged from vehicles or automobiles, and inflammablegases such as CO and CnHm.

2. Description of the Related Art

Various measuring systems and apparatuses have been hitherto suggestedin order to know the concentration of a desired gas component in ameasurement gas.

For example, those known as the method for measuring NOx in ameasurement gas such as combustion gas include a technique in which theNOx-reducing ability of Rh is utilized while using a sensor comprising aPt electrode and an Rh electrode formed on an oxygen ion-conductivesolid electrolyte such as zirconia to measure an electromotive forcegenerated between the both electrodes.

The sensor as described above suffers the following problem. That is,the electromotive force is greatly changed depending on the change inconcentration of oxygen contained in a combustion gas as a measurementgas. Moreover, the change in electromotive force is small with reason,the conventional sensor tends to suffer influence of noise. Further, inorder to bring out the NOx-reducing ability, it is indispensable to usea reducing gas such as CO. For this reason, the amount of produced CO isgenerally smaller than the amount of produced NOx under a lean fuelcombustion condition in which a large amount of NOx is produced.Therefore, the conventional sensor has a drawback in that it isimpossible to perform measurement for a combustion gas produced undersuch a combustion condition.

A system has been disclosed, for example, in Japanese Laid-Open PatentPublication Nos. 63-38154 and 64-39545, in which a pair ofelectrochemical pumping cell and sensor cell comprising Pt electrode andan oxygen ion-conductive solid electrolyte are combined with anotherpair of electrochemical pumping cell and sensor cell comprising Rhelectrode and an oxygen ion-conductive solid electrolyte to measure NOxin accordance with a difference between respective pumping currentvalues.

Further, for example, Japanese Laid-Open Patent Publication Nos.1-277751 and 2-1543 disclose the following method. That is, two pairs ofelectrochemical pumping cells and sensor cells are prepared. Thelimiting pumping current is measured at a partial pressure of oxygen atwhich NOx is not reduced, by using a sensor comprising one of the pairsof pumping cells and sensor cells, while the limiting pumping current ismeasured at a partial pressure of oxygen at which NOx is reduced, byusing a sensor comprising the other pair of pumping cell and sensor cellso that the difference between the limiting pumping currents isdetermined. Besides, the difference in limiting current is measured byusing a sensor comprising a pair of pumping cell and sensor cell, whileswitching the partial pressure of oxygen in a measurement gas between apartial pressure of oxygen at which NOx is reduced and a partialpressure of oxygen at which NOx is not reduced.

SUMMARY OF THE INVENTION

The present invention relates to the gas sensor as described above, anobject of which is to provide a gas sensor which has a self-diagnosisfunction capable of quickly and reliably detecting whether or not thegas sensor has any trouble.

According to the present invention, there is provided a gas sensorcomprising a main pumping means for pumping-processing oxygen containedin a measurement gas introduced from an external space into a processingspace formed and comparted by solid electrolytes contacting with theexternal space; a main pumping control means for comparing a partialpressure of oxygen in the processing space with a first reference valueto control the main pumping means so that the partial pressure of oxygenhas a predetermined value at which a predetermined gas component as ameasurement objective is not decomposable; and an electricsignal-generating conversion means for making conversion into anelectric signal corresponding to an amount of oxygen contained in themeasurement gas after being pumping-processed by the main pumping means;wherein a measurement gas component contained in the measurement gas ismeasured on the basis of the electric signal supplied from the electricsignal-generating conversion means; the gas sensor further comprising anoxygen concentration-detecting means for detecting a concentration ofoxygen contained in the measurement gas after being pumping-processed bythe main pumping means; a correcting control means for correcting andcontrolling the main pumping control means on the basis of a differencebetween a detected value supplied from the oxygenconcentration-detecting means and a second reference value to give aconstant concentration of oxygen contained in the measurement gas afterbeing pumping-processed by the main pumping means; and a self-diagnosismeans for comparing the detected value supplied from the oxygenconcentration-detecting means with a prescribed range to decide whetheror not any trouble occurs, on the basis of a result of the comparison.

According to the present invention, at first, the oxygen, which iscontained in the measurement gas introduced from the external space, ispumping-processed by the main pumping means, and the oxygen is adjustedto have a predetermined concentration. The measurement gas, which hasbeen adjusted for the oxygen concentration by the main pumping means, isintroduced into the electric signal-generating conversion means in thenext step. The electric signal-generating conversion means decomposesthe measurement gas component contained in the introduced measurementgas by means of catalytic action and/or electrolysis, to make conversioninto the electric signal corresponding to the amount of oxygen producedby the decomposition. The measurement gas component contained in themeasurement gas is measured on the basis of the electric signal suppliedfrom the electric signal-generating conversion means.

The detecting operation described above is performed while heating atleast the main pumping means and the electric signal-generatingconversion means to predetermined temperatures by the aid of a heater.Therefore, the amount of the predetermined component is detected highlyaccurately by using the electric signal-generating conversion means.

The predetermined gas component includes, for example, NO, and themeasurement gas component includes, for example, NOx.

When the electric signal-generating conversion means comprises ameasuring pumping means, the measurement gas, which has been adjustedfor the oxygen concentration by the main pumping means, is introducedinto the measuring pumping means.

The measuring pumping means decomposes the measurement gas componentafter being pumping-processed by the main pumping means, by means ofcatalytic action and/or electrolysis, and it pumping-processes oxygenproduced by the decomposition. The predetermined gas componentcorresponding to an amount of oxygen is measured on the basis of apumping current generated in the measuring pumping means in accordancewith the amount of oxygen pumping-processed by the measuring pumpingmeans.

In another embodiment, the electric signal-generating conversion meanscomprises a concentration-detecting means. In this case, the measurementgas, which has been adjusted for the oxygen concentration by the mainpumping means, is introduced into the concentration-detecting means inthe next step. An electromotive force of an oxygen concentration cell isgenerated in the concentration-detecting means, which corresponds to adifference between an amount of oxygen contained in a reference gas andan amount of oxygen produced by decomposition of the predetermined gascomponent contained in the measurement gas. The predetermined gascomponent corresponding to the amount of oxygen is measured on the basisof the electromotive force.

During the period in which the measurement operation is performed forthe predetermined gas component, the concentration of oxygen containedin the measurement gas after being pumping-processed by the main pumpingmeans is detected by the aid of the oxygen concentration-detectingmeans. Further, the main pumping control means is corrected andcontrolled on the basis of the difference between the detected valuesupplied from the oxygen concentration-detecting means and the secondreference value by the aid of the correcting control means. Thus, theconcentration of oxygen contained in the measurement gas after beingpumping-processed by the main pumping means is made constant.

Accordingly, it is possible to avoid the deterioration of accuracy whichwould be otherwise caused by leakage and invasion of oxygen broughtabout by large change in oxygen concentration in the measurement gas.Further, it is possible to avoid the deterioration of accuracy whichwould be otherwise involved in slight decomposition of H₂ O broughtabout by increase in concentration of H₂ O in the measurement gas.Moreover, it is possible to avoid the occurrence of the two types ofdeterioration of accuracy which would be otherwise caused by temperaturechange as well as the occurrence of the two types of deterioration ofaccuracy which would be otherwise caused by deterioration of the mainpumping means.

Further, in the gas sensor according to the present invention, theself-diagnosis means is used to compare the detected value supplied fromthe oxygen concentration-detecting means with the prescribed range sothat it is decided whether or not any trouble occurs, on the basis ofthe result of comparison.

In general, the main pumping means of the gas sensor is operated suchthat the oxygen contained in the measurement gas introduced from theexternal space into the processing space is pumping-processed inaccordance with the control operation effected by the main pumpingcontrol means so that the value of the partial pressure of oxygen in theprocessing space is the predetermined value at which the measurement gascomponent as the measurement objective is not decomposable.

Therefore, if the concentration of oxygen contained in the measurementgas after being pumping-processed by the main pumping means cannot bemade constant although the main pumping control means is corrected andcontrolled by the aid of the correcting control means, namely if thedetected value supplied from the oxygen concentration-detecting meansdoes not arrived at the prescribed range, then the gas sensor is out oforder due to any cause (for example, disconnection of the control systemor the heater or malfunction of the electrode). In the presentinvention, it is decided whether or not any trouble occurs in the gassensor, by utilizing the foregoing principle. Accordingly, the presentinvention makes it possible to promptly and reliably detect whether ornot the gas sensor is in a failure state at present. Therefore, it ispossible to make quick response to maintain and manage the gas sensor.The malfunction of the electrode is exemplified by exhaustion andpeeling-off of the electrode due to thermal damage, and decrease incatalytic activity of the electrode due to, for example, poisoning andclogging.

It is preferable for the gas sensor according to the present inventiondescribed above that the oxygen concentration-detecting means comprisesan auxiliary pumping means for pumping-processing oxygen contained inthe measurement gas after being pumping-processed by the main pumpingmeans, and a value of a pumping current flowing through the auxiliarypumping means is used as the detected value of oxygen concentration.Alternatively, it is preferable that the oxygen concentration-detectingmeans comprises an oxygen partial pressure-detecting means for detectinga difference in partial pressure between oxygen contained in themeasurement gas after being pumping-processed by the main pumping meansand oxygen contained in a reference gas space, and a value of anelectromotive force generated on the basis of the difference in partialpressure is used as the detected value of oxygen concentration.

The correcting control means may comprise a comparing means fordetermining a difference between the detected value supplied from theoxygen concentration-detecting means and the second reference value, anda reference value-correcting means for reflecting the differencesupplied from the comparing means to the first reference value for themain pumping means.

The gas sensor according to the present invention may be constructedsuch that the self-diagnosis means judges that any trouble occurs, whenthe detected value supplied from the oxygen concentration-detectingmeans does not arrive at the prescribed range for a predetermined periodof time.

In this embodiment, the self-diagnosis means comprises a comparing meansfor comparing the detected value supplied from the oxygenconcentration-detecting means with the prescribed range, and amonitoring means for temporarily or periodically monitoring a comparisonoutput supplied from the comparing means and judging that any troubleoccurs, when the comparison output does not arrive at the prescribedrange for a predetermined period of time.

The monitoring means may monitor the comparison output supplied from thecomparing means for the predetermined period of time, upon completion ofa predetermined condition. Alternatively, the monitoring means maymonitor the comparison output supplied from the comparing means atintervals of a certain period of time for the predetermined period oftime. Further alternatively, the monitoring means may be operated inaccordance with a combination of the procedures described above.

As described above, according to the gas sensor concerning the presentinvention, it is possible to promptly and reliably detect whether or notthe gas sensor is in a failure state at present. Therefore, it ispossible to make quick response to maintain and manage the gas sensor.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view illustrating a gas sensor accordingto a first embodiment;

FIG. 2 shows an arrangement of a correcting control system and afeedback control system for a main pumping cell of the gas sensoraccording to the first embodiment;

FIG. 3 shows a block diagram illustrating a specified example of aself-diagnosis unit connected to the gas sensor according to the firstembodiment;

FIG. 4 shows a timing chart illustrating an example of signal processingeffected by the self-diagnosis unit when the gas sensor is normallyoperated;

FIG. 5 shows a timing chart illustrating an example of signal processingeffected by the self-diagnosis unit when the gas sensor is abnormallyoperated;

FIG. 6 shows a cross-sectional view illustrating a first modifiedembodiment of the gas sensor according to the first embodiment;

FIG. 7 shows a cross-sectional view illustrating a second modifiedembodiment of the gas sensor according to the first embodiment;

FIG. 8 shows a cross-sectional view illustrating a gas sensor accordingto a second embodiment;

FIG. 9 shows an arrangement of a correcting control system and afeedback control system for a main pumping cell of the gas sensoraccording to the second embodiment;

FIG. 10 shows a cross-sectional view illustrating a first modifiedembodiment of the gas sensor according to the second embodiment; and

FIG. 11 shows a cross-sectional view illustrating a second modifiedembodiment of the gas sensor according to the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanation will be made below with reference to FIGS. 1 to 11 forseveral illustrative embodiments in which the gas sensor according tothe present invention is applied to gas sensors for measuring oxidessuch as NO, NO₂, SO₂, CO₂, and H₂ O contained in, for example,atmospheric air and exhaust gas discharged from vehicles or automobiles,and inflammable gases such as CO and CnHm.

At first, as shown in FIG. 1, a gas sensor 10A according to the firstembodiment comprises, for example, six stacked solid electrolyte layers12a to 12f composed of ceramics based on the use of oxygenion-conductive solid electrolytes such as ZrO₂. First and second layersfrom the bottom are designated as first and second substrate layers 12a,12b respectively. Third and fifth layers from the bottom are designatedas first and second spacer layers 12c, 12e respectively. Fourth andsixth layers from the bottom are designated as first and second solidelectrolyte layers 12d, 12f respectively.

Specifically, the first spacer layer 12c is stacked on the secondsubstrate layer 12b. The first solid electrolyte layer 12d, the secondspacer layer 12e, and the second solid electrolyte layer 12f aresuccessively stacked on the first spacer layer 12c.

A space (reference gas-introducing space) 14, into which a reference gassuch as atmospheric air to be used as a reference for measuring oxidesis introduced, is formed between the second substrate layer 12b and thefirst solid electrolyte layer 12d, the space 14 being comparted by alower surface of the first solid electrolyte layer 12d, an upper surfaceof the second substrate layer 12b, and side surfaces of the first spacerlayer 12c.

The second spacer layer 12e is interposed between the first and secondsolid electrolyte layers 12d, 12f. First and second diffusionrate-determining sections 16, 18 are also interposed between the firstand second solid electrolyte layers 12d, 12f.

A first chamber 20 for adjusting the partial pressure of oxygen in ameasurement gas is formed and comparted by a lower surface of the secondsolid electrolyte layer 12f, side surfaces of the first and seconddiffusion rate-determining sections 16, 18, and an upper surface of thefirst solid electrolyte layer 12d. A second chamber 22 for finelyadjusting the partial pressure of oxygen in the measurement gas andmeasuring oxides, for example, nitrogen oxides (NOx) in the measurementgas is formed and comparted by a lower surface of the second solidelectrolyte layer 12f, a side surface of the second diffusionrate-determining section 18, a side surface of the second spacer layer12e, and an upper surface of the first solid electrolyte layer 12d.

The external space communicates with the first chamber 20 via the firstdiffusion-rate determining section 16, and the first chamber 20communicates with the second chamber 22 via the second diffusionrate-determining section 18.

The first and second diffusion-rate determining sections 16, 18 givepredetermined diffusion resistances to the measurement gas to beintroduced into the first and second chambers 20, 22 respectively. Eachof the first and second diffusion-rate determining sections 16, 18 canbe formed as a passage composed of, for example, a porous material (forexample, a porous compact composed of ZrO₂ or the like), or a small holehaving a predetermined cross-sectional area so that the measurement gasmay be introduced. Alternatively, each of the first and seconddiffusion-rate determining sections 16, 18 may be constructed by a gaplayer or a porous layer produced by printing. In this embodiment, thecomparative magnitude does not matter between the respective diffusionresistances of the first and second diffusion rate-determining sections16, 18. However, it is preferable that the diffusion resistance of thesecond diffusion rate-determining section 18 is larger than that of thefirst diffusion rate-determining section 16.

The atmosphere in the first chamber 20 is introduced into the secondchamber 22 under the predetermined diffusion resistance via the seconddiffusion rate-determining section 18.

An inner pumping electrode 24 having a substantially rectangular planarconfiguration and composed of a porous cermet electrode is formed on theentire lower surface portion for forming the first chamber 20, of thelower surface of the second solid electrolyte layer 12f. An outerpumping electrode 26 is formed on a portion corresponding to the innerpumping electrode 24, of the upper surface of the second solidelectrolyte layer 12f. An electrochemical pumping cell, i.e., a mainpumping cell 28 is constructed by the inner pumping electrode 24, theouter pumping electrode 26, and the second solid electrolyte layer 12finterposed between the both electrodes 24, 26.

A desired control voltage (pumping voltage) Vp0 is applied between theinner pumping electrode 24 and the outer pumping electrode 26 of themain pumping cell 28 by the aid of an external variable power source 30to allow a pumping current IpO to flow in a positive or negativedirection between the outer pumping electrode 26 and the inner pumpingelectrode 24. Thus, the oxygen in the atmosphere in the first chamber 20can be pumped out to the external space, or the oxygen in the externalspace can be pumped into the first chamber 20.

A reference electrode 32 is formed on a lower surface portion exposed tothe reference gas-introducing space 14, of the lower surface of thefirst solid electrolyte layer 12d. An electrochemical sensor cell, i.e.,a controlling oxygen partial pressure-detecting cell 34 is constructedby the inner pumping electrode 24, the reference electrode 32, thesecond solid electrolyte layer 12f, the second spacer layer 12e, and thefirst solid electrolyte layer 12d.

The controlling oxygen partial pressure-detecting cell 34 is operated asfollows. That is, an electromotive force (voltage) VO is generatedbetween the inner pumping electrode 24 and the reference electrode 32 onthe basis of a difference in oxygen concentration between the atmospherein the first chamber 20 and the reference gas (atmospheric air) in thereference gas-introducing space 14. The partial pressure of oxygen inthe atmosphere in the first chamber 20 can be detected by using theelectromotive force VO.

That is, the voltage VO generated between the reference electrode 32 andthe inner pumping electrode 24 is an electromotive force of the oxygenconcentration cell generated on the basis of the difference between thepartial pressure of oxygen of the reference gas introduced into thereference gas-introducing space 14 and the partial pressure of oxygen ofthe measurement gas in the first chamber 20. The voltage VO has thefollowing relationship known as the Nernst's equation.

    VO=RT/4F·ln (P1(O.sub.2)/P0(O.sub.2))

R: gas constant;

T: absolute temperature;

F: Faraday constant;

P1(O₂): partial pressure of oxygen in the first chamber 20;

P0(O₂): partial pressure of oxygen in the reference gas.

Therefore, the partial pressure of oxygen in the first chamber 20 can bedetected by measuring the voltage VO generated on the basis of theNernst's equation, by using a voltmeter 36. The detected value of thepartial pressure of oxygen is used to control the pumping voltage Vp0 ofthe variable power source 30 by the aid of a feedback control system 38.Specifically, the pumping operation effected by the main pumping cell 28is controlled so that the partial pressure of oxygen in the atmospherein the first chamber 20 has a predetermined value which is sufficientlylow to control the partial pressure of oxygen in the second chamber 22in the next step.

Especially, in this embodiment, when the amount of oxygen pumped out bythe main pumping cell 28 is changed, and the oxygen concentration in thefirst chamber 20 is changed, then the terminal voltage between thereference electrode 32 and the inner pumping electrode 24 of the mainpumping cell 28 is changed without any time delay (changed in realtime). Therefore, it is possible to effectively suppress the oscillationphenomenon which would be otherwise caused in the feedback controlsystem 38.

Each of the inner pumping electrode 24 and the outer pumping electrode26 is composed of an inert material having a low catalytic activity onNOx such as NO contained in the measurement gas introduced into thefirst chamber 20. Specifically, the inner pumping electrode 24 and theouter pumping electrode 26 may be composed of a porous cermet electrode.In this embodiment, the electrode is composed of a metal such as Pt anda ceramic such as ZrO₂. Especially, it is necessary to use a materialwhich has a weak reducing ability or no reducing ability with respect tothe NO component in the measurement gas, for the inner pumping electrode24 disposed in the first chamber 20 to make contact with the measurementgas. It is preferable that the inner pumping electrode 24 is composedof, for example, a compound having the perovskite structure such as La₃CuO₄, a cermet comprising a ceramic and a metal such as Au having a lowcatalytic activity, or a cermet comprising a ceramic, a metal of the Ptgroup, and a metal such as Au having a low catalytic activity. When analloy composed of Au and a metal of the Pt group is used as an electrodematerial, it is preferable to add Au in an amount of 0.03 to 35% byvolume of the entire metal component.

Specifically, as shown in FIG. 2, a circuit system (feedback controlsystem) 38 for performing the feedback control comprises a firstdifferential amplifier 40 for determining a difference between anelectric potential Va of the reference electrode 32 and an electricpotential Vb of the inner pumping electrode 24, and amplifying thedetermined difference with a predetermined gain to make an output as ameasured voltage Vc; a second differential amplifier 42 for determininga difference between the output Vc of the first differential amplifier40 and a first reference voltage Vd, and amplifying the determineddifference with a predetermined gain to make an output; and asignal-amplifying system 44 composed of a one-stage or multi-stageamplifier for amplifying the output Ve of the second differentialamplifier 42 with a predetermined gain. In this embodiment, the wiringconnection is made so that the output of the signal-amplifying system44, i.e., the voltage Vp0 is supplied to the outer pumping electrode 26of the main pumping cell 28, and the inner pumping electrode 24 isgrounded. The signal-amplifying system 44, which is disposed at thefinal stage, serves to efficiently operate the main pumping cell 28 byamplifying the signal having the small level supplied from the previousstage with the predetermined gain.

Accordingly, at first, the measurement gas is introduced into the firstchamber 20 via the first diffusion rate-determining section 16. Theelectric potential Va of the reference electrode 32 and the electricpotential Vb of the inner pumping electrode 24 at that time are suppliedto respective input terminals of the first differential amplifier 40.The first differential amplifier 40 outputs the difference (measuredvoltage) Vc between the electric potentials Va, Vb. The measured voltageVc is applied, for example, to an inverting terminal of the seconddifferential amplifier 42 disposed at the downstream stage. The seconddifferential amplifier 42 determines the difference between the measuredvoltage Vc supplied to the inverting terminal and the first referencevoltage Vd supplied to a non-inverting terminal. The voltage signal Ve,which is obtained by amplifying the determined difference with thepredetermined gain, is outputted from an output terminal of the seconddifferential amplifier 42. The voltage signal Ve is amplified with thepredetermined gain by the signal-amplifying system 44 disposed at thedownstream stage, and an obtained voltage is supplied as the pumpingvoltage Vp0 to the outer pumping electrode 26 of the main pumping cell28. In this embodiment, the inner pumping electrode 24 has the groundelectric potential (0 V). Therefore, the voltage between the bothelectrodes 24, 26 of the main pumping cell 28 is equivalent to thepumping voltage Vp0 supplied from the signal-amplifying system 44 afterall.

Therefore, the main pumping cell 28 pumps out or pumps in oxygen in anamount corresponding to the level of the pumping voltage Vp0, of themeasurement gas introduced into the first chamber 20. The oxygenconcentration in the first chamber 20 is subjected to feedback controlto give a predetermined level by repeating the series of operationsdescribed above.

On the other hand, as shown in FIG. 1, an auxiliary pumping electrode 50having a substantially rectangular planar configuration and composed ofa porous cermet electrode is formed on the entire lower surface portionfor forming the second chamber 22, of the lower surface of the secondsolid electrolyte layer 12f. An auxiliary electrochemical pumping cell,i.e., an auxiliary pumping cell 52 is constructed by the auxiliarypumping electrode 50, the reference electrode 32, and the first solidelectrolyte layer 12d.

It is preferable that the auxiliary pumping electrode 50 is composed ofa material having a weak reducing ability or no reducing ability withrespect to the NO component contained in the measurement gas, forexample, a compound having the perovskite structure such as La₃ CuO₄, acermet comprising a ceramic and a metal having a low catalytic activitysuch as Au, or a cermet comprising a ceramic, a metal of the Pt group,and a metal having a low catalytic activity such as Au, in the samemanner as the inner pumping electrode 24 of the main pumping cell 28.Further, when an alloy comprising Au and a metal of the Pt group is usedas an electrode material, it is preferable to add Au in an amount of0.03 to 35% by volume of the entire metal components. A desired constantvoltage Vp1 is applied between the reference electrode 32 and theauxiliary pumping electrode 50 of the auxiliary pumping cell 52 by theaid of an external power source 54. Thus, the oxygen in the atmospherein the second chamber 22 can be pumped out to the referencegas-introducing space 14. Accordingly, the partial pressure of oxygen inthe atmosphere in the second chamber 22 is allowed to have a low valueof partial pressure of oxygen at which the measurement of the amount ofthe objective component is not substantially affected, under thecondition in which the measurement gas component (NOx) is notsubstantially reduced or decomposed. In this embodiment, owing to theoperation of the main pumping cell 28 for the first chamber 20, thechange in amount of oxygen introduced into the second chamber 22 isgreatly reduced as compared with the change in the measurement gas.Accordingly, the partial pressure of oxygen in the second chamber 22 isaccurately controlled to be constant.

In the gas sensor 10A according to the first embodiment, a detectingelectrode 56 having a substantially rectangular planar configuration andcomposed of a porous cermet electrode is formed at a portion separatedfrom the second diffusion rate-determining section 18, on an uppersurface portion for forming the second chamber 22, of the upper surfaceof the first solid electrolyte layer 12d. An electrochemical pumpingcell, i.e., a measuring pumping cell 58 is constructed by the detectingelectrode 56, the reference electrode 32, and the first solidelectrolyte layer 12d.

The detecting electrode 56 is composed of a porous cermet comprisingzirconia as a ceramic and Rh as a metal capable of reducing NOx as themeasurement gas component. Accordingly, the detecting electrode 56functions as a NOx-reducing catalyst for reducing NOx existing in theatmosphere in the second chamber 22. Further, the oxygen in theatmosphere in the second chamber 22 can be pumped out to the referencegas-introducing space 14 by applying a constant voltage Vp2 between thedetecting electrode 56 and the reference electrode 32 by the aid of a DCpower source 60. The pumping current Ip2, which is allowed to flow inaccordance with the pumping operation performed by the measuring pumpingcell 58, is detected by an ammeter 62.

The constant voltage (DC) power source 60 can apply a voltage of amagnitude to give a limiting current to the pumping for oxygen producedduring decomposition in the measuring pumping cell 58.

The gas sensor 10A according to the first embodiment further comprises aheater 64 for generating heat in accordance with electric power supplyfrom the outside. The heater 64 is embedded in a form of beingvertically interposed between the first and second substrate layers 12a,12b. The heater 64 is provided in order to increase the conductivity ofoxygen ion. A ceramic layer 66 composed of alumina or the like is formedto cover upper and lower surfaces of the heater 64 so that the heater 64is electrically insulated from the substrate layers 12a, 12b.

As shown in FIG. 1, the heater 64 is arranged over the entire portionranging from the first chamber 20 to the second chamber 22. Accordingly,each of the first chamber 20 and the second chamber 22 is heated to apredetermined temperature. Simultaneously, each of the main pumping cell28, the controlling oxygen partial pressure-detecting cell 34, theauxiliary pumping cell 52, and the measuring pumping cell 58 is alsoheated to a predetermined temperature and maintained at thattemperature.

The gas sensor 10A according to the first embodiment further comprises acorrecting control system 70 for correcting and controlling the feedbackcontrol system 38 of the main pumping cell 28 on the basis of the valueof the pumping current Ip1 flowing through the auxiliary pumping cell52.

As shown in FIG. 2, the correcting control system 70 comprises aresistor R1 inserted and connected between the DC power source 54 andthe reference electrode 32 for converting the pumping current Ip1flowing through the auxiliary pumping cell 52 into a voltage signal Vf,an amplifier 72 for amplifying the voltage signal Vf with apredetermined gain to make an output as an auxiliary pumping voltage Vg,an electrolytic capacitor C, and a resistor R2. The correcting controlsystem 70 further comprises an integrating circuit (low-pass filter) 74for stably operating the correcting control system 70 connected to thefeedback control system 38, a third differential amplifier 76 fordetermining a difference between an output voltage Vh supplied from theintegrating circuit 74 and a second reference voltage Vi and amplifyingthe determined difference with a predetermined gain, and a resistor R3for converting an output current supplied from the third differentialamplifier 76 into a voltage signal (correcting voltage) Vj to besuperimposed on the first reference voltage Vd used for the feedbackcontrol system 38. The second reference voltage Vi is set to be avoltage corresponding to the desired (constant) oxygen concentration inthe second chamber 22.

In this description, the relationship between the pumping currentflowing through the auxiliary pumping cell 52 and the voltage appearingon the resistor R1 is conveniently defined as follows.

When the oxygen concentration in the second chamber 22 is higher than aprescribed concentration (represented by a concentration higher than adesired constant level to some extent), and a large amount of oxygen ispumping-processed by the auxiliary pumping cell 52, then a large amountof pumping current flows through the resistor R1. Under this condition,the voltage is increased in the positive direction. The value of thepumping current is decreased as the oxygen concentration in the secondchamber 22 is gradually lowered in accordance with the pumping processeffected by the main pumping cell 28 and the auxiliary pumping cell 52.The voltage Vf is also decreased during this process.

The operation of the correcting control system 70 will now be brieflyexplained. At first, the pumping current Ip1 flowing through theauxiliary pumping cell 52, i.e., the oxygen concentration in the secondchamber 22 is detected by the aid of the resistor R1 inserted andconnected between the reference electrode 32 and the DC power source 54of the auxiliary pumping cell 52, which is outputted as the voltagesignal Vf corresponding to the oxygen concentration.

The voltage signal Vf is amplified with the predetermined gain to givethe auxiliary pumping voltage Vg by means of the amplifier 72 disposedat the downstream stage. The auxiliary pumping voltage Vg is processedby the integrating circuit 74 disposed at the downstream stage to givethe output voltage Vh which is inputted into the third differentialamplifier 76 disposed at the downstream stage.

The integrating circuit 74 has its circuit constants (resistance valueand capacitance value) which are set to give a time constantcorresponding to the delay time depending on the diffusion resistance ofthe second diffusion rate-determining section 18. Accordingly, theintegrating operation is added to the control operation effected by thecorrecting control system 70. The oscillation phenomenon in thecorrecting control system 70, which would be otherwise caused bydisturbance or the like, is effectively avoided. Thus, the controloperation is stably performed.

The third differential amplifier 76 determines the difference betweenthe second reference voltage Vi and the output voltage Vh supplied fromthe integrating circuit 74 disposed at the upstream stage. The current(current in the positive or negative direction) corresponding to thedetermined difference is allowed to flow on the output side. The currentflows through the resistor R3. Voltage drop occurs during this processto make conversion into the correcting voltage Vj corresponding to thecurrent value. The correcting voltage Vj is superimposed on the firstreference voltage Vd.

The correcting operation performed by the correcting control system 70for the first reference voltage Vd allows the second differentialamplifier 42 of the feedback control system 38 to determine a differencebetween the voltage Vc based on the partial pressure of oxygen in thefirst chamber 20 and a new reference voltage {first reference voltageVd+(difference between auxiliary pumping voltage Vh and second referencevoltage Vi)}. The oxygen concentration in the second chamber 22 isreflected (superimposed) as the correcting voltage Vj onto the firstreference voltage Vd. That is, the second differential amplifier 42 hasa function to vary and modulate the oxygen concentration in the firstchamber 20 depending on the pumping current Ip1 flowing through theauxiliary pumping cell 52.

The correcting operation, which is effected for the first referencevoltage Vd by the correcting control system 70, provides a constantoxygen concentration in the second chamber 22. Accordingly, it ispossible to avoid the deterioration of accuracy which would be otherwisecaused by leakage and invasion of oxygen brought about by large changein oxygen concentration in the measurement gas. Further, it is possibleto avoid the deterioration of accuracy which would be otherwise involvedin slight decomposition of H₂ O brought about by increase inconcentration of H₂ O in the measurement gas. Moreover, it is possibleto avoid the occurrence of the two types of deterioration of accuracywhich would be otherwise caused by temperature change as well as theoccurrence of the two types of deterioration of accuracy which would beotherwise caused by deterioration of the main pumping cell 28.Especially, as shown in FIG. 1, the gas sensor 10A according to thefirst embodiment includes a self-diagnosis unit 100 for monitoring thecondition of the gas sensor 10A, the self-diagnosis unit 100 beingconnected downstream from the auxiliary pumping cell 52.

Specifically, as shown in FIG. 2, an output line of the amplifier 72 isbranched into two. One output line is connected to one terminal of theresistor R2 of the integrating circuit, and the other output line isconnected to the self-diagnosis unit 100.

As shown in FIG. 3, the self-diagnosis unit 100 comprises a comparatorcircuit 102 for comparing the level of the voltage signal Vg suppliedfrom the amplifier 72 with a predetermined prescribed range (upper limitlevel Ea to lower limit level Eb), a clock-generating unit 104 forgenerating a predetermined clock Pc, a trigger pulse-generating circuit106 for generating a trigger pulse signal Pt on the basis of an input ofan instruction signal Sg supplied, for example, from an unillustratedmicrocomputer installed outside, a window pulse-generating circuit 108for generating a window pulse Pw having a predetermined pulse width onthe basis of an input of the trigger pules signal Pt supplied from thetrigger pulse-generating circuit 106, a judging circuit 110 for judgingwhether or not the level of the voltage signal Vg arrives at theprescribed range (level Ea to Eb) within the pulse width of the windowpulse Pw outputted from the window pulse-generating circuit 108, adecoder 112 for analyzing a result of judgement supplied from thejudging circuit 110 to make an output as a display control signal, and adisplay controller 114 for outputting, to a display unit 116, a displaysignal or display data corresponding to an attribute of the controlsignal supplied from the decoder 112.

The comparator circuit 102 comprises a first comparator 120 forcomparing the level of the voltage signal Vg supplied from the amplifier72 with the upper limit level Ea, a second comparator 122 for comparingthe level of the voltage signal Vg supplied from the amplifier 72 withthe lower limit level Eb, and a decoder 124 for performing predeterminedlogical operation {for example, exclusive OR (XOR)} for the outputs fromthe first and second comparators 120, 122 to make an output as acomparison result signal Sh.

The voltage signal Vg1 outputted from the first comparator 120 is at alow level if the level of the voltage signal Vg is higher than the upperlimit level Ea. The voltage signal Vg1 is at a high level if the levelof the voltage signal Vg is lower than the upper limit level Ea.

The voltage signal Vg2 outputted from the second comparator 122 is at alow level if the level of the voltage signal Vg is higher than the lowerlimit level Eb. The voltage signal Vg2 is at a high level if the levelof the voltage signal Vg is lower than the lower limit level Eb.

The comparison result signal Sh outputted from the decoder 124 is at alow level if both of the voltage signals Vg1, Vg2 are at high levels orlow levels (namely if the level of the voltage signal Vg is without theprescribed range). The comparison result signal Sh is at a high level ifthe voltage signal Vg1 is at a high level and the voltage signal Vg2 isat a low level (namely if the level of the voltage signal Vg is withinthe prescribed range).

On the other hand, the trigger pulse-generating circuit 106 is in anenable state, for example, on the basis of the input of the instructionsignal Sg from the outside, and it generates one trigger pulse Pt, forexample, at an initial rising timing of an clock Pc. Thereafter, thetrigger pulse-generating circuit 106 generates the trigger pulse Ptevery time when a predetermined number of clocks are counted.

The window pulse-generating circuit 108 is in an enable state on thebasis of the input of the trigger pulse Pt supplied from the triggerpulse-generating circuit 106, and it generates, for example, one windowpulse Pw which rises at the initial rising timing of the clock Pc andwhich falls at a point of time at which a predetermined number of clocksare counted (see FIGS. 4 and 5).

The judging circuit 110 outputs two types of judgement signals (firstand second judgement signals Si1, Si2) depending on the change in levelof the window pulse Pw and the output signal Sh from the comparatorcircuit 102.

As shown in FIG. 4, for example, the first judgement signal Si1 is at alow level if the output signal Sh from the comparator circuit 102 is ata low level at the point of time of rising of the window pulse Pw, andit is at a high level if the output signal Sh from the comparatorcircuit 102 is at a high level within the pulse width of the windowpulse Pw. Therefore, if the output signal from the comparator circuit102 is not at the high level within the pulse width of the window pulsePw, that is, if the level of the voltage signal Vf is not within theprescribed range, then the first judgement signal Si1 maintains the lowlevel.

As shown in FIG. 5, for example, the second judgement signal Si2 is atthe high level at the point of time of completion of the window pulse Pw(at the point of falling thereof) if the first judgement signal Si1 isat the low level.

The decoder 112 outputs a control signal (for example, a low levelsignal) for indicating "normal" to the display controller 114 disposeddownstream if the first and second judgement signals Si1, Si2 are at thehigh level and the low level respectively. The decoder 112 outputs acontrol signal (for example, a high level signal) for indicating"abnormal" to the display controller 114 disposed downstream if thefirst and second judgement signals Si1, Si2 are at the low level and thehigh level respectively.

The display controller 114 outputs, to the display unit 116 disposeddownstream, information indicating "normals", for example, display datafor message or symbol to indicate "normal" if the control signal fedfrom the decoder 112 indicates "normal". When the display unit 116 is,for example, an LED (light emitting diode), the display controller 114outputs, for example, a signal indicating light-out.

On the other hand, the display controller 114 outputs informationindicating "abnormal", for example, display data for message or symbolto indicate "abnormal" if the control signal fed from the decoder 112indicates "abnormal". When the display unit 116 is, for example, an LED(light emitting diode), the display controller 114 outputs, for example,a signal indicating light-up.

If the control signal indicating "abnormal" is supplied from the decoder112 disposed upstream, the display controller 114 outputs a disablesignal Sj to the trigger pulse-generating circuit 106 so that thetrigger pulse-generating circuit 106 is in a stopped state.

The gas sensor 10A according to the first embodiment is basicallyconstructed as described above. Next, its function and effect,especially function and effect of the self-diagnosis unit 100 will beexplained.

At first, when the power source is turned on for the apparatus installedwith the gas sensor 10A, the initial operation is performed in theapparatus. The initial operation includes electric power application tothe heater 64 of the gas sensor 10A.

At a point of time after passage of a predetermined period of time (forexample, a period of time for completing the warming-up process for thegas sensor 10A) from the point of time of the electric power applicationto the heater 64, the microcomputer (not shown) outputs the instructionsignal Sg to the trigger pulse-generating circuit 106 of theself-diagnosis unit 100. When the apparatus for installing the gassensor 10A therein is an automobile, the point of time of the completionof the warming-up process indicates a point of time at which the watertemperature arrives at a predetermined value.

From the point of time at which the instruction signal Sg is suppliedfrom the microcomputer (not shown) to the self-diagnosis unit 100, theself-diagnosis unit 100 starts monitoring for the gas sensor 10A, i.e.,monitoring for the pumping current Ip1 flowing through the auxiliarypumping cell 52. In the first embodiment, the monitoring is performedfor the voltage signal Vf which appears in the resistor R1 in accordancewith the pumping current Ip1 flowing through the auxiliary pumping cell52. As shown in FIG. 4, if the value of the pumping current flowingthrough the auxiliary pumping cell 52 (level of the voltage signal Vf)arrives at the prescribed range (within the range from the upper limitlevel Ea to the lower limit level Eb) within the predetermined period oftime (within the pulse width of the window pulse Pw), the first andsecond judgement signals Si1, Si2 outputted from the judging circuit 110are at the high level and the low level respectively. Therefore, thecontrol signal indicating "normal" is outputted from the decoder 112. Asa result, the display unit 116 makes a display indicating "normal".

After that, the instruction signal Sg is periodically supplied from themicrocomputer (not shown) to the self-diagnosis unit 100. Self-diagnosisfor the gas sensor 10A is performed every time when the instructionsignal Sg is supplied.

On the other hand, as shown in FIG. 5, if the level of the voltagesignal Vg does not arrive at the prescribed range after passage of thepredetermined period of time, the judging circuit 110 outputs the fistjudgement signal Si1 at the low level and the second judgement signalSi2 at the high level respectively. Accordingly, the control signalindicating "abnormal" is outputted from the decoder 112, and the displayunit 116 makes a display indicating "abnormal". Upon the judgement ofabnormality, the disable signal Sj is outputted from the displaycontroller 114 to the trigger first embodiment, it is possible topromptly and reliably detect whether or not the gas sensor 10A is in afailure state at present. Therefore, it is possible to make quickresponse to maintain and manage the gas sensor 10A (including responseto legislation).

The trouble or failure of the gas sensor 10A includes, for example,failure of the main pumping cell 28 or the auxiliary pumping cell 52itself, disconnection of the feedback control system 38 or the heater64, and malfunction of the electrode. The malfunction of the electrodeis exemplified by exhaustion and peeling-off of the electrode due tothermal damage, and decrease in catalytic activity of the electrode dueto, for example, poisoning and clogging.

The self-diagnosis unit 100 judges that any trouble occurs when thevoltage signal Vg supplied from the amplifier 72 does not arrive at theprescribed range even after passage of the predetermined period of time.Alternatively, the self-diagnosis unit 100 may judge that any troubleoccurs, at a point of time at which the voltage signal Vg supplied fromthe amplifier 72 is deviated from the prescribed range. In thisembodiment, the wiring connection is preferably made as follows. Thatis, the comparison result signal Sh supplied from the decoder 124 in thecomparator circuit 102 is directly inputted into the display controller114 (see two-dot chain line). Further, the circuit is constructed andassembled such that the display controller 114 outputs information toindicate pulse-generating circuit 106. The process for judging thetrouble to be performed by the self-diagnosis unit 100 thereafter iscompleted. The display indicating "abnormal" is made until the resetinput is made for the display unit 116.

In general, the main pumping cell 28 of the gas sensor 10A is operatedas follows. That is, the oxygen, which is contained in the measurementgas introduced from the external space into the first chamber 20, ispumping-processed in accordance with the control operation effected bythe feedback control system 38 as described above. Thus, the value ofthe partial pressure of oxygen in the first chamber 20 is allowed tohave the predetermined value at which the NO component as themeasurement objective is not decomposable.

Therefore, if the oxygen concentration in the second chamber 22 cannotarrive at the prescribed level, namely if the pumping current Ip1flowing through the auxiliary pumping cell 52 does not arrive at theprescribed range (the voltage signal Vg does not arrive at theprescribed range), although the feedback control system 38 is subjectedto the correcting control by the aid of the correcting control system70, then the gas sensor 10A has any trouble due to any cause. In thefirst embodiment, it is decided whether or not any trouble occurs in thegas sensor 10A by utilizing the foregoing principle.

As a result, in the gas sensor 10A according to the "normal" to thedisplay unit 116 disposed downstream if the inputted comparison resultsignal Sh is at the high level, while the display controller 114 outputsinformation to indicate "abnormal" to the display unit 116 disposeddownstream if the inputted comparison result signal Sh is at the lowlevel.

Alternatively, the display controller 114 may be circuited andconstructed as follows. That is, in the initial stage, it is monitoredwhether or not the voltage signal Vg arrives at the prescribed rangewithin the predetermined period through the passage of comparatorcircuit 102→judging circuit 110→decoder 112→display controller 114.After it is judged that no trouble occurs, it is monitored whether ornot the voltage signal Vg arrives at the prescribed range within thepredetermined period in real time through the passage of comparatorcircuit 102→display controller 114 (see two-dot chain line).

Next, two modified embodiments of the gas sensor 10A according to thefirst embodiment will be described with reference to FIGS. 6 and 7.Components or parts corresponding to those shown in FIG. 1 aredesignated by the same reference numerals, duplicate explanation ofwhich will be omitted.

At first, as shown in FIG. 6, a gas sensor 10Aa according to the firstmodified embodiment is constructed in approximately the same manner asthe gas sensor 10A according to the first embodiment. However, theformer is different from the latter in that a measuring oxygen partialpressure-detecting cell 170 is provided in place of the measuringpumping cell 58.

The measuring oxygen partial pressure-detecting cell 170 comprises adetecting electrode 172 formed on an upper surface portion for formingthe second chamber 22, of the upper surface of the first solidelectrolyte layer 12d, the reference electrode 32 formed on the lowersurface of the first solid electrolyte layer 12d, and the first solidelectrolyte layer 12d interposed between the both electrodes 172, 32.

In this embodiment, an electromotive force (electromotive force of anoxygen concentration cell) corresponding to the difference in oxygenconcentration between the atmosphere around the detecting electrode 172and the atmosphere around the reference electrode 32 is generatedbetween the reference electrode 32 and the detecting electrode 172 ofthe measuring oxygen partial pressure-detecting cell 170.

Therefore, the partial pressure of oxygen in the atmosphere around thedetecting electrode 172, in other words, the partial pressure of oxygendefined by oxygen produced by reduction or decomposition of themeasurement gas component (NOx) is detected as a voltage value bymeasuring the electromotive force generated between the detectingelectrode 172 and the reference electrode 32 by using a voltmeter 174.

The gas sensor 10Aa according to the first modified embodiment alsocomprises the feedback control system 38, the auxiliary pumping cell 52,the correcting control system 70, and the self-diagnosis unit 100, inthe same manner as the gas sensor 10A according to the first embodiment.

Therefore, it is also possible for the gas sensor 10Aa according to thefirst modified embodiment to avoid the deterioration of accuracy whichwould be otherwise caused by leakage and invasion of oxygen broughtabout by large change in oxygen concentration in the measurement gas.Further, it is possible to avoid the deterioration of accuracy whichwould be otherwise involved in slight decomposition of H₂ O broughtabout by increase in concentration of H₂ O in the measurement gas, inthe same manner as the gas sensor 10A according to the first embodiment.Moreover, it is possible to promptly and reliably detect whether or notthe gas sensor 10Aa is in a failure state at present. Therefore, it ispossible to make quick response to maintain and manage the gas sensor10Aa.

Next, a gas sensor 10Ab according to the second modified embodimentshown in FIG. 7 is constructed in approximately the same manner as thegas sensor 10Aa according to the first modified embodiment. However, theformer is different from the latter in that both of the measuringpumping cell 58 and the measuring oxygen partial pressure-detecting cell170 are provided, and the value of partial pressure of oxygen (voltageV2) detected by the measuring oxygen partial pressure-detecting cell 170is used to control the pumping voltage Vp2 of a variable power source60A of the measuring pumping cell 58 by the aid of the feedback controlsystem 130.

In this embodiment, the measuring pumping cell 58 comprises thedetecting electrode 172, the inner pumping electrode 24, the first solidelectrolyte layer 12d between the both electrodes 172, 24, the secondspacer layer 12e, and the second solid electrolyte layer 12f. The oxygenin the atmosphere in the second chamber 22 can be pumped out to thefirst chamber 20 by applying the voltage Vp2 by the aid of the variablepower source 60A.

The gas sensor 10Ab according to the second modified embodiment alsocomprises the feedback control system 38, the auxiliary pumping cell 52,the correcting control system 70, and the self-diagnosis unit 100, inthe same manner as the gas sensor 10A according to the first embodiment.Therefore, it is possible to avoid the deterioration of accuracy whichwould be otherwise caused by leakage and invasion of oxygen broughtabout by large change in oxygen concentration in the measurement gas.Further, it is possible to avoid the deterioration of accuracy whichwould be otherwise involved in slight decomposition of H₂ O broughtabout by increase in concentration of H₂ O in the measurement gas.Moreover, it is possible to promptly and reliably detect whether or notthe gas sensor 10Ab is in a failure state at present. Therefore, it ispossible to make quick response to maintain and manage the gas sensor10Ab.

In the gas sensors according to the first embodiment (including theseveral modified embodiments) 10A, 10Aa, 10Ab, the output signal Vg,which is supplied from the amplifier 72 for amplifying, with thepredetermined gain, the voltage signal Vf obtained by converting thepumping current Ip1 into the voltage, is inputted into theself-diagnosis unit 100. Alternatively, the output signal Vh afterpassing through the integrating circuit 74 may be inputted into theself-diagnosis unit 100. In this embodiment, the signal Vh, from whichthe high pass noise is removed, is inputted into the self-diagnosis unit100. Therefore, it is possible to perform the self-diagnosis moreaccurately.

Next, a gas sensor 10B according to the second embodiment will beexplained with reference to FIG. 8. Components or part corresponding tothose shown in FIG. 1 are designated by the same reference numerals,duplicate explanation of which will be omitted.

As shown in FIG. 8, the gas sensor 10B according to the secondembodiment is constructed in approximately the same manner as the gassensor 10A according to the first embodiment. However, the former isdifferent from the latter in that a correcting oxygen partialpressure-detecting cell 140 is provided in place of the auxiliarypumping cell 52, and a correcting control system 146 is provided forcorrecting and controlling the feedback control system 38 of the mainpumping cell 28 on the basis of a voltage value V1 detected by thecorrecting oxygen partial pressure-detecting cell 140.

The correcting oxygen partial pressure-detecting cell 140 comprises ameasuring electrode 142 formed on an upper surface portion for formingthe second chamber 22, of the upper surface of the first solidelectrolyte layer 12d, the reference electrode 32 formed on the lowersurface of the first solid electrolyte layer 12d, and the first solidelectrolyte layer 12d interposed between the both electrodes 142, 32.

In this embodiment, an electromotive force (electromotive force of anoxygen concentration cell) corresponding to the difference in oxygenconcentration between the atmosphere in the second chamber 22 and theatmosphere around the reference electrode 32 is generated between thereference electrode 32 and the measuring electrode 142 of the correctingoxygen partial pressure-detecting cell 140.

Therefore, the partial pressure of oxygen in the atmosphere around themeasuring electrode 142, in other words, the partial pressure of oxygenin the second chamber 22 is detected as a voltage value by measuring theelectromotive force generated between the measuring electrode 142 andthe reference electrode 32 by using a voltmeter 144.

On the other hand, as shown in FIG. 9, the correcting control system 146comprises a fourth differential amplifier 150 for determining adifference between a difference (measured voltage Vm) between anelectric potential of the measuring electrode 142 and the groundelectric potential and a difference (reference voltage Vn) between anelectric potential of the reference electrode 32 and the ground electricpotential and amplifying the determined difference with a predeterminedgain to make an output as a detection voltage V1 corresponding to thepartial pressure of oxygen in the second chamber 22, a thirddifferential amplifier 76 for determining a difference between thedetection voltage V1 supplied from the fourth differential amplifier 150and a second reference voltage Vi and amplifying the determineddifference with a predetermined gain, an electrolytic capacitor C, and aresistor R2. The correcting control system 146 further comprises anintegrating circuit (low-pass filter) 74 for stably operating thecorrecting control system 146 connected to the feedback control system38, and a resistor R3 for converting an output current supplied from theintegrating circuit 74 into a voltage signal (correcting voltage Vj) tobe superimposed on the first reference voltage Vd used for the feedbackcontrol system 38.

In this description, the relationship between the value V1 and thevoltage appearing on the resistor R1 is conveniently defined as follows.

When the oxygen concentration in the second chamber 22 is higher than aprescribed concentration (represented by a concentration higher than adesired constant level to some extent), the voltage detected by thecorrecting oxygen partial pressure-detecting cell 140 is also increasedin the positive direction. The detection voltage V1 is decreased as theoxygen concentration in the second chamber 22 is gradually lowered inaccordance with the pumping process effected by the main pumping cell28.

The operation of the correcting control system 146 will now be brieflyexplained. At first, the fourth differential amplifier 150 is used todetermine the difference between the reference voltage Vn and themeasured voltage Vm obtained by the correcting oxygen partialpressure-detecting cell 140. The difference is extracted as thedetection voltage V1.

The third differential amplifier 76 is used to determine the differencebetween the second reference voltage Vi and the detection voltage V1supplied from the fourth differential amplifier 150. The current(current in the positive or negative direction) corresponding to thedetermined difference flows on the output side. The current flowsthrough the integrating circuit 74 disposed downstream, and it flowsthrough the resistor R3. Voltage drop occurs during this process, andthe current is converted into the correcting voltage Vj corresponding tothe current value. The correcting voltage Vj is superimposed on thefirst reference voltage Vd.

The integrating circuit 74 has its circuit constants (resistance valueand capacitance value) which are set to give a time constantcorresponding to the delay time depending on the diffusion resistance ofthe second diffusion rate-determining section 18, in the same manner asthe gas sensor 10A according to the first embodiment. Accordingly, theintegrating operation is added to the control operation effected by thecorrecting control system 146. The oscillation phenomenon in thecorrecting control system 146, which would be otherwise caused bydisturbance or the like, is effectively avoided. Thus, the controloperation is stably performed.

The correcting operation performed by the correcting control system 146for the first reference voltage Vd allows the second differentialamplifier 42 of the feedback control system 38 to determine a differencebetween the voltage Vc based on the partial pressure of oxygen in thefirst chamber 20 and a new reference voltage {first reference voltageVd+(difference between detection voltage V1 and second reference voltageVi)}. The oxygen concentration in the second chamber 22 is reflected(superimposed) as the correcting voltage Vj onto the first referencevoltage Vd. That is, the second differential amplifier 42 has a functionto vary and modulate the oxygen concentration in the first chamber 20depending on the detection voltage V1 detected by the correcting oxygenpartial pressure-detecting cell 140. The correcting operation, which iseffected for the first reference voltage Vd by the correcting controlsystem 146, provides a constant oxygen concentration in the secondchamber 22. Accordingly, it is possible to avoid the deterioration ofaccuracy which would be otherwise caused by leakage and invasion ofoxygen brought about by large change in oxygen concentration in themeasurement gas. Further, it is possible to avoid the deterioration ofaccuracy which would be otherwise involved in slight decomposition of H₂O brought about by increase in concentration of H₂ O in the measurementgas. Moreover, it is possible to avoid the occurrence of the two typesof deterioration of accuracy which would be otherwise caused bytemperature change as well as the occurrence of the two types ofdeterioration of accuracy which would be otherwise caused bydeterioration of the main pumping cell 28.

Especially, as shown in FIG. 9, in the gas sensor 10B according to thesecond embodiment, an output line of the fourth differential amplifier150 Is branched into two. One output line is connected to an invertinginput terminal of the third differential amplifier 76, and the otheroutput line is connected to a self-diagnosis unit 100. Theself-diagnosis unit 100 is constructed in the same manner as theself-diagnosis unit 100 shown in FIG. 3 except that the signal, which Isinputted into the self-diagnosis unit 100 via the other output line, isthe detection voltage V1 supplied from the fourth differential amplifier150. Therefore, detailed explanation therefor will be omitted.

Therefore, as for the gas sensor 10B according to the second embodiment,it is possible to promptly and reliably detect whether or not the gassensor 10B is in a failure state at present. Therefore, it is possibleto make quick response to maintain and manage the gas sensor 10B.

Next, two modified embodiments of the gas sensor 10B according to thesecond embodiment will be described with reference to FIGS. 10 and 11.Components or parts corresponding to those shown in FIG. 8 aredesignated by the same reference numerals, duplicate explanation ofwhich will be omitted.

At first, as shown in FIG. 10, a gas sensor 10Ba according to the firstmodified embodiment is constructed in approximately the same manner asthe gas sensor 10B according to the second embodiment. However, theformer is different from the latter in that a measuring oxygen partialpressure-detecting cell 170 is provided in place of the measuringpumping cell 58.

The measuring oxygen partial pressure-detecting cell 170 is the same asthe measuring oxygen partial pressure-detecting cell 170 of the gassensor 10Aa according to the first modified embodiment concerning thefirst embodiment shown in FIG. 6. Therefore, detailed explanationtherefor will be omitted.

The gas sensor 10Ba according to the first modified embodiment alsocomprises the feedback control system 38, the correcting oxygen partialpressure-detecting cell 140, the correcting control system 146, and theself-diagnosis unit 100, in the same manner as the gas sensor 10Baccording to the second embodiment. Accordingly, it is also possible toavoid the deterioration of accuracy which would be otherwise caused byleakage and invasion of oxygen brought about by large change in oxygenconcentration in the measurement gas. Further, it is possible to avoidthe deterioration of accuracy which would be otherwise involved inslight decomposition of H₂ O brought about by increase in concentrationof H₂ O in the measurement gas. Moreover, it is possible to promptly andreliably detect whether or not the gas sensor 10Ba is in a failure stateat present. Therefore, it is possible to make quick response to maintainand manage the gas sensor 10Ba.

Next, a gas sensor 10Bb according to the second modified embodimentshown in FIG. 11 is constructed in approximately the same manner as thegas sensor 10Ba according to the first modified embodiment. However, theformer is different from the latter in that both of the measuringpumping cell 58 and the measuring oxygen partial pressure-detecting cell170 are provided, and the value of partial pressure of oxygen detectedby the measuring oxygen partial pressure-detecting cell 170 is used tocontrol the pumping voltage Vp2 of a variable power source 60A of themeasuring pumping cell 58 by the aid of the feedback control system 130,in the same manner as the gas sensor 10Ab according to the secondmodified embodiment concerning the first embodiment shown in FIG. 7.

The gas sensor 10Bb according to the second modified embodiment alsocomprises the feedback control system 38, the correcting oxygen partialpressure-detecting cell 140, the correcting control system 146, and theself-diagnosis unit 100, in the same manner as the gas sensor 10Baccording to the second embodiment. Therefore, it is possible to avoidthe deterioration of accuracy which would be otherwise caused by leakageand invasion of oxygen brought about by large change in oxygenconcentration in the measurement gas. Further, it is possible to avoidthe deterioration of accuracy which would be otherwise involved inslight decomposition of H₂ O brought about by increase in concentrationof H₂ O in the measurement gas. Moreover, it is possible to promptly andreliably detect whether or not the gas sensor 10Bb is in a failure stateat present. Therefore, it is possible to make quick response to maintainand manage the gas sensor 10Bb.

In the gas sensors according to the second embodiment (including theseveral modified embodiments) 10B, 10Ba, 10Bb, the voltage signal V1,which is supplied from the fourth differential amplifier 150, isinputted into the self-diagnosis unit 100. Alternatively, the voltagesignal after passing through the integrating circuit 74 may be inputtedinto the self-diagnosis unit 100. In this embodiment, the voltagesignal, from which the high pass noise is removed, is inputted into theself-diagnosis unit 100. Therefore, it is possible to perform theself-diagnosis more accurately.

In the respective second modified embodiments 10Ab, 10Bb of the gassensors 10A, 10B according to the first and second embodiments, the oneelectrode of the measuring pumping cell 58 is the inner pumpingelectrode 24 of the main pumping cell 28. Alternatively, the oneelectrode may be the outer pumping electrode 26. In this case, theoxygen in the atmosphere in the second chamber 22 is pumped out to theexternal space.

In the gas sensors according to the first and second embodiments(including several modified embodiments) 10A, 10Aa, 10Ab, 10B, 10Ba,10Bb, the arrangement as shown in FIG. 3 is adopted for theself-diagnosis unit 100. However, this arrangement is persistentlyillustrative. The gas sensor of the present invention can be constructedby using various combinations of digital and analog circuits.

The gas sensors according to the embodiments described above aredirected to NOx as the measurement gas component. However, the presentinvention is also effectively applicable to the measurement of boundoxygen-containing gas components such as H₂ O and CO₂ other than NOx, inwhich the measurement is affected by oxygen existing in the measurementgas.

It is a matter of course that the present invention is not limited tothe embodiments described above, which may be constructed in othervarious forms without deviating from the gist or essentialcharacteristics of the present invention.

What is claimed is:
 1. A gas sensor comprising:a main pumping means forpumping-processing oxygen contained in a measurement gas introduced froman external space into a processing space formed by solid electrolytescontacting with said external space; a main pumping control means forcomparing a partial pressure of oxygen in said processing space with afirst reference value to control said main pumping means so that saidpartial pressure of oxygen has a predetermined value at which apredetermined gas component as a measurement objective is notdecomposable; and an electric signal-generating conversion means fordecomposing said predetermined gas component contained in saidmeasurement gas after being pumping-processed by said main pumpingmeans, by the aid of a catalytic action and/or electrolysis for creatingan electric signal corresponding to an amount of oxygen produced by saiddecomposition wherein: said predetermined gas component contained insaid measurement gas is measured on the basis of said electric signalsupplied from said electric signal-generating conversion means, said gassensor further comprising:an oxygen concentration-detecting means fordetecting a concentration of oxygen contained in said measurement gasafter being pumping-processed by said main pumping means; a correctingcontrol means for correcting and controlling said main pumping controlmeans on the basis of a difference between a detected voltage or currentvalue proportional to the concentration of oxygen as measured by theoxygen concentration detecting means supplied from said oxygenconcentration-detecting means and a second reference value to give aconstant concentration of oxygen contained in said measurement gas afterbeing pumping-processed by said main pumping means; and a self-diagnosismeans for comparing said detected voltage or current value supplied fromsaid oxygen concentration-detecting means with a prescribed range todecide whether or not any abnormal condition occurs, on the basis of aresult of said comparison.
 2. The gas sensor according to claim 1,wherein:said electric signal-generating conversion means comprises ameasuring pumping means for decomposing said predetermined gas componentcontained in said measurement gas after being pumping-processed by saidmain pumping means, by means of catalytic action and/or electrolysis,and pumping-processing oxygen produced by said decomposition; and saidmeasurement gas component contained in said measurement gas is measuredon the basis of a pumping current flowing through said measuring pumpingmeans in accordance with said pumping process effected by said measuringpumping means.
 3. The gas sensor according to claim 1, wherein:saidelectric signal-generating conversion means comprises an oxygen partialpressure-detecting means for decomposing said predetermined gascomponent contained in said measurement gas after beingpumping-processed by said main pumping means, by means of catalyticaction, and generating an electromotive force corresponding to adifference between an amount of oxygen contained in a reference gas andan amount of oxygen produced by said decomposition; and said measurementgas component contained in said measurement gas is measured on the basisof said electromotive force detected by said concentration-detectingmeans.
 4. The gas sensor according to claim 1, wherein said oxygenconcentration-detecting means comprises an auxiliary pumping means forpumping-processing oxygen contained in said measurement gas after beingpumping-processed by said main pumping means, and a value of a pumpingcurrent flowing through said auxiliary pumping means is used as saiddetected value of oxygen concentration.
 5. The gas sensor according toclaim 1, wherein said oxygen concentration-detecting means comprises anoxygen partial pressure-detecting means for detecting a difference inpartial pressure between oxygen contained in said measurement gas afterbeing pumping-processed by said main pumping means and oxygen containedin a reference gas space, and a value of an electromotive forcegenerated on the basis of said difference in partial pressure is used assaid detected value of oxygen concentration.
 6. The gas sensor accordingto claim 1, wherein said correcting control means comprises:a comparingmeans for determining a difference between said detected value suppliedfrom said oxygen concentration-detecting means and said second referencevalue; and a reference value-correcting means for reflecting saiddifference supplied from said comparing means to said first referencevalue for said main pumping means.
 7. The gas sensor according to claim1, wherein said self-diagnosis means judges that any abnormal conditionoccurs, when said detected voltage or current value supplied from saidoxygen concentration detecting means does not arrive at said prescribedrange for a predetermined period of time.
 8. The gas sensor according toclaim 1, wherein said self-diagnosis means comprises:a comparing meansfor comparing said detected value supplied from said oxygenconcentration-detecting means with said prescribed range; and amonitoring means for temporarily or periodically monitoring a comparisonoutput supplied from said comparing means and judging that anyabnormality occurs, when said comparison output does not arrive at saidprescribed range for a predetermined period of time.
 9. The gas sensoraccording to claim 8, wherein said monitoring means monitors saidcomparison output supplied from said comparing means at intervals of acertain period of time for said predetermined period of time.
 10. Thegas sensor according to claim 8, wherein said monitoring means monitorssaid comparison output supplied from said comparing means for saidpredetermined period of time, which period ends upon completion of apredetermined condition.